Temperature dependence of different resistors and diodes

What you can learn about

Principle:The temperature dependence of anelectrical parameter (e.g. resistance,conducting-state voltage, blockingvoltage) of different components isdetermined. To do this, the immersion probe set is immersed in a waterbath and the resistance is measuredat regular temperature intervals.

Connecting cord, l = 750 mm, red

Connecting cord, l = 750 mm, blue

Cobra3 Basic Unit

Cobra3 Function generator module

RS232 data cable

14602.00

PC, Windows 95 or higher

Complete Equipment Set, Manual on CD-ROM included

Temperature dependence of differentresistors and diodesP24104 01/15

Diagram of resistances.

Tasks:1. Measurement of the temperaturedependence of the resistance ofdifferent electrical components.2. Measurement of the temperaturedependence of the conductingstate voltage of semiconductingdiodes.3. Measurement of the temperaturedependence of the voltage in theZener and the avalanche effects.

Connecting cord, l = 750 mm, red

Connecting cord, l = 750 mm, blue

07362.0107362.04

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Tasks1. Measurement of the temperature dependence of the resistance of different electrical components.2. Measurement of the temperature dependence of the conducting state voltage of semiconducting diodes.3. Measurement of the temperature dependence of the voltage in the Zener and the avalanche effects.Set-up and procedure1. Place the immersion probe set, which is enclosed in awatertight plastic bag, into the water bath. The resistancevalues for the PTC, NTC, metallic film and carbon filmresistors, as well as the Cu and CuNi wire resistors, can bemeasured directly with the digital hand multimeter (circuitdiagram, Fig. 2). To do this, connect the multimeter to theground jack, which is connected to all the components,and the jack located under the symbol corresponding tothe respective component. Note the different resistancevalues, and plot them as a function of temperature.

Fig. 1. Photograph of the experimental set-up.

Temperature dependence of different resistors and diodes

1. In copper wire the free path of the electrons in the electron

vapour, which contribute to charge transport, becomesshorter with increasing temperature. The change in resistance can be clearly seen: the resistance increases. Theresult is a positive temperature coefficientCu = 5.3 10-3 /K

Fig. 2

2. In order to measure the conducting-state voltage of the

semiconducting diodes, connect them to a voltage of 10 V.Connect a 4.7 resistor in series with the component. Seta voltage of 10 V on the universal power supply, and adjustthe current limiter to its maximum value. Measure the voltage parallel to the component. Note the conducting-statevoltage corresponding to the respective temperatures.

The resistance of the CuNi wire is nearly constant over the

measured range. This is in accordance with Mathies rule,which states that Rtot = R20 + R(T). The change in theresistance with the temperature is very slight in the measured temperature range. Consequently, the absolute resistance (R20) is predominant. This experiment provides anegative temperature coefficient ofCuNi = -1.4 10-4 /KIn the carbon-layer resistor, the absolute resistance is veryhigh to begin with. The change with the temperature is, asis the case with CuNi, small and has practically no effect.A negative temperature coefficient resultsCu = -2.3 10-3 /KThe metallic layer resister also has a relatively high absolute resistance at 20 C. And the change in the measuredtemperature range is even lower than for carbon. Thus, thetemperature coefficient approaches zero.met = 1 0The two NTC and PTC resistors consist of alloys.Depending on their compositions, great changes in resistance can be realised in a small temperature range. Thecurves that are recorded in this experiment can no longerbe considered linear. They serve only to illustrate the behaviour of NTC and PTC resistors.

Fig. 3

3. Also measure the blocking voltage for the Zener and avalanche effects with the set-up illustrated in Fig. 3. However,the diodes have already been wired in the blocking directionthrough their placement in the immersion probe set.

Theory and Evaluation

In restricted temperature ranges the change in the resistanceof the electrical components can be assumed to be linear. Inthese regions, the general formula for the dependence of theresistance on the temperature is valid

one can see that the intrinsic conductivity of the semiconductor thus increases. The mobility indeed decreases withincreasing temperature, but the increase in the charge carrier density compensates for this effect. A definite drop inresistance is observed; this allows one to infer that there isa negative temperature coefficient. Through the calculationwith the above-mentioned formula for the temperaturedependence, rearranged for the voltage Up, the followingvalues are obtained.Si = -3.4 10-3 /KGe = -4.6 10-3 /K

Fig. 6: Diagram of resistances.

3. At low voltages, around 3 V, a Zener breakdown occurs in

Z diodes. As a result of the strong electric field, electronhole pairs are spontaneously generated in the inner electron shells in the barrier-layer zone. Under the influence ofthe field charge carrier, they cross the barrier layer. A higher temperature increases the energy of the bound chargecarriers. As a consequence, the Zener effect can occur atlower voltages. In the avalanche effect, the charge carriersare accelerated by the electric field to such a great degreethat they in turn release other charge carriers on collidingwith other atoms, which in turn are accelerated. The higher temperature shortens the free path, so that the voltagemust increase with the temperature in order to continue torelease charge carriers. From the calculations, the following values result for :ZPD2.7 = -7.3 10-4 /KZPD2.8 = +4.5 10-4 /KLiterature values:ZPD2.7 = -9-4 10-4 /KZPD6.8 = +2+7 10~4 /K